And no sandboxing isn’t the solution here. Facebook will just find ways around it. They did some pretty egregious bullshit and had consumers side-loading privacy-violating apps via their corporate account. Apple very publicly revoked their certificate for it. Without the walled garden, Apple would have no leverage to stop bad actors like Facebook.
And avoiding widely used social apps isn’t a good solution either. The network effects effectively removes me from communities that use it.